Fix /proc/acpi/alarm BCD alarm encodings

This fixes some totally illogical and wrong code that converts things to
and from BCD mode essentially randomly, does math on values in BCD mode
etc etc.  Introduce a few helper functions to make it a bit more obvious
what is going on, and make sure that we always do all the arithmetic
(and anythign else, for that matter) in binary, not BCD.

Tested by Mark Lord, who found the problem originally, and also pushed
the patch back and reminded me about it.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>